Anisotropic magnetoresistance and planar Hall ... Anisotropic magnetoresistance and planar Hall effect

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  • Turk J Phys (2020) 44: 77 – 84 © TÜBİTAK doi:10.3906/fiz-1910-15

    Turkish Journal of Physics

    http :// journa l s . tub i tak .gov . t r/phys i c s/

    Research Article

    Anisotropic magnetoresistance and planar Hall effect in magnetoresistive NiFe/Pt thin film

    Erdem DEMİRCİ∗ Department of Physics, Faculty of Science, Gebze Technical University, Kocaeli, Turkey

    Received: 16.10.2019 • Accepted/Published Online: 28.01.2020 • Final Version: 12.02.2020

    Abstract: The magnetoresistive NiFe/Pt sensor was fabricated by using standard microlithography and sputter depo- sition techniques and then the magnetic and magnetotransport properties were investigated. The magnetic properties of the sample show that the easy axis of magnetization prefers a predefined direction along the Hall bar. Moreover, the sample has uniaxial magnetic anisotropy due to the growth-induced magnetic anisotropy. On the other hand, the Anisotropic magnetoresistance (AMR) and the Planar Hall effect (PHE) of the sample were investigated in detail be- tween 0°and 360°. The angle-dependent AMR and PHE signals at the in-plane magnetic field of constant magnitude intersect at certain angles. At these angles, the presence of unusual peaks and valleys was observed. This is proof that AMR and PHE curves had strongly affected each other at certain angles.

    Key words: Magnetoresistive sensor, planar Hall effect, anisotropic magnetoresistance, NiFe

    1. Introduction In recent years, the magnetoresistive sensors based on anisotropic magnetoresistive (AMR) have attracted great attention owing to their high sensitivity at weak magnetic fields, the flexibility of the design, and compatibility with standard microelectronics technology [1–5]. The magnetoresistive sensors are commonly used for magnetic bead detection [6], biosensors and biochips [7, 8], applications to microcompass [9], and flexible magnetic sensors [10, 11]. The physical phenomenon of the magnetoresistance effect is related to the change in the resistance of the magnetic conductor under external magnetic field [12]. The change in the resistance (R) is experimentally expressed as

    R=R0+∆R cos 2 θ (1)

    Here, R0 is the electrical resistance in the absence of a magnetic field, ∆R is the difference in the resistance of the magnetic conductor and θ is the angle between the current (I) and magnetization (M) .

    It is known that the AMR effect arises from the anisotropic scattering of conductive electrons in a ferromagnetic material [13]. In this phenomenon, the resistance is at the maximum when M and I are parallel (R//) or it is at minimum when M and I are perpendicular with each other (R⊥) . The AMR effect with in-plane magnetization is described by Eq. (2) [13]. However, unlike the AMR in longitudinal geometry, the magnetoresistance measurement in transverse geometry causes the electric field perpendicular to the current. The phenomenon in transverse geometry is called the planar Hall effect (PHE) and it is expressed by Eq. (3) [13]. Although the PHE output voltage is very low compared to the AMR output voltage, the planar Hall effect ∗Correspondence: e.demirci@gtu.edu.tr

    This work is licensed under a Creative Commons Attribution 4.0 International License. 77

    https://orcid.org/0000-0001-5626-5292

  • DEMİRCİ/Turk J Phys

    is considerably reduced the temperature drift and increased the resolution of magnetoresistive sensors [14]. For a single domain ferromagnetic conductor, electric fields (E) of AMR and PHE are expressed as follows:

    EAMR=jρ⊥+j(ρ||−ρ⊥) cos2 θ (2)

    EPHE= j(ρ||−ρ⊥) sin θ cos θ (3)

    Here, ρ// and ρ⊥ are the resistivities for the current parallel and perpendicular to the magnetization orien- tations, respectively. According to Eq. 3, PHE voltage is proportional to the sin2θ angle between M and I . Therefore, PHE voltage is extremely sensitive rather than AMR voltage to detect small deviations in the magnetization from the current direction [15, 16]. Also, the background signals existing in the AMR are not observed in the PHE. By considering these disadvantages of AMR compared to PHE, the magnetoresistive sensor based on AMR requires high sensibility to the deviation of magnetoresistance for novel technology.

    In the present work, the magnetoresistive Py (10 nm)/Pt (5 nm) sensor was fabricated with Hall bar and continuous film structures on SiO2 /Si. Then, AMR and PHE measurements of the sensor were performed in an applied external magnetic field at a different angle to compare the effect of their signal responses on each other. To observe a small deviation of magnetoresistance based on current was examined in detail by using PHE and AMR techniques.

    2. Experiment and discussion 2.1. Fabrication The magnetoresistive sensor based on AMR was designed and fabricated using standard microlithography and sputter deposition techniques. First, Hall bar and continuous patterns were realized on SiO2 /Si substrates by optical lithography. Before the lithography processes, the SiO2/Si substrate was cleaned by sequentially rinsing in acetone, isopropanol, and deionized water. After that, a positive photoresist (AZ 1505) was coated onto the cleaned SiO2 /Si substrate by using a spin coater. To form a uniform photoresist with a thickness of 500 µm, coating processes were carried out at 4000 rpm for 50 s. The photoresist was baked on the hot plate at 110 °C for 60 s. Then, it was exposed at constant intensity ultraviolet light (365 nm, i -line) for 0.5 s and developed by using AZ 726 MIF for 60 s. After the patterning process, Py (10 nm)/ Pt (5 nm) layers were deposited on SiO2 /Si by using magnetron sputtering. Py and Pt layers were deposited at room temperature by using 20 W (RF) and 10 W (DC) powers, respectively. The base pressure level of the sputter chamber was 6 ×10−9 mbar and the working pressure was kept at 5 ×10−3 mbar during the deposition. After that, the sample was soaked in acetone for 10 min and residual photoresist on SiO2/Si substrate was lifted off. The microscope image of the sample consists of Hall bar and continuous parts are as shown in Figure 1.

    Figure 2 shows the measurement geometry for the magnetotransport experiments. In this geometry, the easy axis of the sample is parallel to I . Here, θ is the angle between I and the M , and θH is the angle between I and the external magnetic field (Hext) . Magnetoresistive experiments were performed by using a magnetotransport system. A typical current strength was supplied by a Keithley 2400 source meter and the current was fixed at 50 µA for all magnetoresistive experiments. The magnetic field-dependent voltage was measured by a Keithley 2182 nano voltmeter. The angle-dependent magnetoresistive experiments were carried out between 0°and 360°by using an integrated stepper motor. During the magnetoresistive experiments, a constant current was passed through the long direction of the Hal bar between 1 and 6 terminals for all

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    magnetotransport measurements. Then, the AMR voltage between 3 and 4 terminals (V3− 4) and PHE voltage between 2 and 3 terminals (V2− 3) were recorded by using nanovolt meter while the in-plane magnetic field was swept. The distance is 200 µm between the magnetoresistance terminals (V3− 4) , and 25 µm between the planar Hall terminals (V2− 3) . The long axis of the sample in the Hall bar is prepared with 8:1 aspect ratio to induce shape anisotropy.

    Figure 1. Photograph of the sample mounted on a printed circuit board (PCB). The sample consists of Hall bar (top) and continuous film (bottom). The contact pads and PCB terminals were connected with gold wires by using wire bonder. The inset is a microscope top-view image of the magnetoresistive Py/Pt sensor.

    Figure 2. Schematic representation of the Hall bar geom- etry with the 6 terminals. The width of the Hall bar is 25 µm and the length is 200 µm. A constant current of 50 µA was passed through 1–6 terminals by using Keithley 2400 source meter. The voltage is measured 2–3 terminals for PHE and 4–5 terminals for AMR by using Keithley 2182 nanovoltmeter.

    3. Results and discussion Magnetic hysteresis loops of the sample were measured by using a magneto-optical Kerr effect (MOKE) at room temperature. First, angle-dependent longitudinal MOKE (L-MOKE) measurements were carried out from 0°to 360°by using a stepper motor. L-MOKE hysteresis loops of the Si/ SiO2 / Py/ Pt were used for the determination of the angular dependence of the normalized remanent magnetization (MR /MS) as shown in Figure 3. The sample exhibits in-plane uniaxial magnetic anisotropy. The growth-induced anisotropy is the major cause of this behavior of the sample since direction of magnetization is closely related to the growth conditions in the magnetron sputtering systems. During deposition, the substrate is deposited with atoms from an oblique angle; thus, magnetization can be oriented in a specific direction. Here, the easy axis of the magnetization prefers to extend along the Hall bar of the sample due to the growth-induced magnetic anisotropy [17,18]. In this way, M and Iwill be parallel to each other. Figure 3 insets show hysteresis loops of the sample taken at the angles of 0°and 90°. Easy and hard axes of the sample are at φ = 0°(blue) and φ = 90°(red), respectively.

    In magnetoresistive experiments, two types of measurements were performed to compare the relationship between the resistance and applied magnetic field. The first is the dependence of AMR and PHE voltages on the magnetic field while the sample is at a fixed angle. The second is the dependence of the AMR and PHE voltages on the angle at the fixed magnetic field.

    Fig